The other option -which I'm testing out this week- is to have the ALT+TAB switch between apps, and ALT+| switch between windows of the same app. It sounds complicated but it works surprisingly well.

Use case A: You're on Firefox and want to switch to Thunderbird: ALT+TAB.

Use case B: You're on a Calc window and want to switch to another Calc window: ALT+|.

In B, you could use the old way of ALT+TAB, where it cycles through all your windows, but it can take a while to do that if you have more than a few windows open. With this thing, you always know where you want to go, and choose the quicker way to get there.

I choose | because it's just above TAB, so my brain and my finger don't need to move very far.

To enable this: System Settings > Keyboard > Shortcuts > Navigation > Switch windows of an application (it's just underneath Switch applications, which is how I found it).